In Northern Ireland this last sheaf of wheat was called "Calacht". It was ritually cut by the harvesters as seen on this photo entitled "Cutting the ‘Calacht’" by WA Green. Image sourced from the National Museum of Northern Ireland.
In "Folk-lore - A Quarterly Review", Volume 25 published in 1914, we can read a short description of this custom which once marked the end of the grain harvest in Ireland:
Harvest Meal from Co. Antrim. Notice the straw "Calacht" (Cailleach) over the table. Photo by WA Green. Image sourced from the National Museum of Northern Ireland.
The same tradition was once common in Scotland too. There the object made from the last sheaf was called "cailleach-bhuaineadh" (‘the old woman of the harvest’). Here is an example of the cailleach from Scotland.
The reason people gave for this custom can be found on the website "Digital Archive of Scottish Gaelic"
Cailleach, The Hag represented Mother Earth in her winter form. At the end of the harvest (literally means autumn) her spirit (ability to produce food) was spent, and the last of it was preserved in this effigy for the next year. Pic: Winter, Roman mosaic
During the winter, as the grain from the autumn harvest was ground into flour, the Ugly Old Hag of Winter transformed into the Beautiful Young Maiden of Spring full of reproductive energy again. Pic: Slovenian folk painting depicting "Babji mlin" (Grandmother's mill)